One of the most common tasks involving circuit design relates to the design and evaluation of data communication paths. Binary encoded serial data communication technology has become the predominant form of interconnect for many communication standards and is being chosen almost exclusively for new applications that require high data rates. A typical serial data communication channel employs a data transmission scheme that utilizes a single electrical or optical path over which the data is sent. An appropriate data encoding scheme provides for the extraction of the data signal at the receiver. The use of a single signal transmission path avoids the problems of inter-channel synchronization encountered when using multiple, synchronous data channels.
Although the use of a single signal transmission path avoids the multiple channel synchronization issue, it also forces the data rate of that single channel to a maximum. Since the time between symbols is inversely proportional to the data rate, ever increasing data rates cause the time separating data symbols to decrease. As inter-symbol spacing decreases, the symbols begin to interfere with one another causing the determination of the data symbol to become difficult, ultimately resulting in bit errors. The presence of noise in the system compounds the problem of determining the correct value of a data symbol.
The vast majority of current high speed serial data communications standards employ Non Return to Zero (NRZ) encoding with a small minority utilizing Return to Zero (RZ) and other schemes. NRZ and RZ encoding are binary encoding schemes. The NRZ encoding scheme is very straightforward in that a high amplitude represents a “1” value and a low amplitude represents a “0” value. For RZ encoding a “1” value is represented by a positive going pulse and a low value is represented by a low amplitude. In either of these encoding schemes, the locations of transitions from one amplitude level to another can give a good estimation of the bit error rate of the system. Since low system bit error rate is the goal of all data communication systems, it is also the primary metric provided by all serial data communication measurement systems.
The primary types of equipment used to measure the fidelity of serial communication systems are oscilloscopes, time interval analyzers and bit error rate test sets. Each of these measurement methods generally relate the location of the transitions in time relative to the ideal sampling time along some reference amplitude threshold.
In the case of oscilloscopes, the waveform amplitude is sampled at known times. There are two types of oscilloscopes that are employed for this type of analysis.
The first type is a real time or digitizing sampling oscilloscope. This oscilloscope sequentially samples the waveform at a very high rate (e.g., a rate high enough so that each data symbol is sampled a few times). However, having finite memory, the real time oscilloscope can only digitize the waveform for a number of samples equal to the memory depth of the oscilloscope. This results in a real time amplitude record of the waveform for a period of time. A real time record enables a number of analysis techniques that take into account the displacement of transitions relative to an ideal transition location. The ideal transition location is derived from the entire sequence of transition locations and can incorporate numerous mathematical techniques to emulate receiver functionality without the need for hardware to emulate the receiver.
Deficiencies related to real time oscilloscopes have to do with the rate at which they can digitize waveforms. As was stated earlier, at the highest data rates only a few samples are obtained for each data symbol. This necessitates the need to estimate the transition points based on interpolation. As will be appreciated, the resulting error is oftentimes significant. Another shortcoming of real time oscilloscopes is that they have bandwidth limitations that are significant relative to the data rates used in serial data communication signals which again result in errors significant to measurement accuracy.
The second type of oscilloscope is an equivalent time oscilloscope which measures the waveform in relation to a repeating trigger event. This measurement is under-sampled in the sense that only one out of a plurality of triggers is selected for instigation of an amplitude meaurement. The benefits of this type of measurement are that the sampling hardware can measure the signal with very high bandwidth (e.g., greater than 60 gigahertz) resulting in very accurate reproduction of all of the waveform—unlike the real time oscilloscopes which are quite limited in bandwidth.
One major deficiency of equivalent time measurements is related to the under-sampling. In an under-sampled measurement technique, the measurements can only be related to one another via the fact that they were initiated by similar trigger events. No other information is available regarding the relationship of one sample to another. Consequently if an ideal sampling time is desired for analysis purposes, either a reference “clock” signal must be supplied to the oscilloscope, usually in the form of a trigger, or one must be extracted from the incoming waveform with clock recovery hardware. However, even with the presence of the reference clock signal, the temporal measurement to measurement information available with a real time oscilloscope cannot be produced by the equivalent time oscilloscope. This significantly limits the types of analyses that are supported by the data, and consequently limits the types of diagnostic conclusions that can be reached.
Time interval analyzers have circuitry that directly measures the transition points avoiding the estimation error encountered by real time oscilloscopes due to interpolation. They are, however, an under-sampled measurement system and suffer from similar limitations as the equivalent time oscilloscopes.
Bit error rate test sets have clock recovery built in and actually sample the incoming waveform at an estimated ideal sampling time and threshold. Since the result of this sampling is merely digital and not parametric, only a bit error rate is determined. In order to estimate bit error rate, the sampling point must be moved and consequent bit error rates must be measured at each new sampling point. This method is also typically very slow relative to other methods.
Shortcomings of the current measurement methods are summarized:
1. Real time oscilloscopes                a. Systems do not measure threshold transition locations directly, must estimate them using interpolation.        b. Bandwidth limitations relative to data rate reduce accuracy and limit data rate measurement capability.        
2. Equivalent time oscilloscope                a. Under-sampling results in a non-real time record which cannot support as rich an analysis as a real time record.        b. Low measurement rate, long acquisition time        c. Trigger is required, but not always available.        d. May require clock recovery.        
3. Time interval analyzer                a. Under-sampling results in a non-real time record which cannot support as rich an analysis as a real time record.        b. Limited amplitude analysis capability        c. May require clock recovery.        
4. Bit error rate test set                a. Very limited parametric measurement capability        b. Very slow acquisition        
In view of the foregoing, there arises a need for an improved threshold crossing time measurement and recording method and apparatus. The following invention addresses and helps to solve and/or minimize the shortcomings of the prior art.